The present invention is directed to a method and apparatus for melting glass and more particularly to a method and apparatus for the production of optical glass having a reduced variation in the refractive index.
In general, during the continuous production of a small amount of glass such as optical glass, a raw material is first melted in a melting tank to form the molten glass having a uniform refractive index. The molten glass is first transferred to a refining tank and bubbles and striae are removed and is then transferred to a working tank where the temperature of the molten glass is adjusted to a suitable level for the flow of the glass through an outlet. In this way, the melting tank, the refining tank and the working tank are separated from each other and connected by a connecting pipe so tnat the molten glass in each tank does not mix with the molten glass in another tank thereby increasing the production efficiency.
In the prior art system shown in FIG. 1, the melting tank 1, the refining tank 3 and the working tank 4 are connected to each other by connecting pipes 11 and 12. The raw material is continuously fed to the melting tank 1 where it is melted to form molten glass. The molten glass is transferred to the refining tank 3 which is provided with a stirring means where the bubbles and striae are removed. The molten glass is then sent to the working tank 4 which is also provided with a stirring means 13 in which the molten glass is completely freed of striae and at the same time is regulated to the proper temperature for subsequent flow through the outlet pipe 5. While the refractive index of the molten glass in the melting tank can be made uniform or homogenized over a sufficient period of time, there is an unevenness in the raw materials being placed in the melting tank during a continuous feed arrangement so that the refractive indices of the glass products obtained will be uneven or different due to the different raw materials.
Another prior art method involves the intermittent or batch production of glass wherein the raw material is placed in a crucible and melted to form molten glass. The homogenization of the refractive index of the molten glass, the removal of bubbles and striae and the regulation of the temperature to a suitable level for outflow are carried out successively in the same crucible.
Of the above-described conventional prior art melting methods, the continuous melting method is advantageous in that the production efficiency is very high since all of the tanks are separated from each other and the molten glass in each tank is prevented from mixing with the molten glass of another tank. However, because of the serious variations in the refractive index due to changes in the composition of the raw material, some of the glass produced by this system cannot be used as optical glass. On the other hand, in the case of the intermittent or batch method, the production efficiency is low even though the uniformity of the refractive index is assured. This leads to an increase in production costs. Furthermore, in the case of special glass, a tank made of an expensive metal, such as platinum or a platinum alloy, is often used which further increases the cost.